Multipart MIME user-data format not supported

[Tim Black]

Some may call this a feature request, but I call it a bug, at least in the docs, since they claim Subiquity supports cloud-init user-data, implying it supports ALL of cloud-init user-data, including all standard formats. Here I request that subiquity add support for standard Multipart MIME user-data format, or to more clearly document precisely what kind of user-data it _does_ support and why, or to correct my logic and assumptions and show me how to properly specify and configure and use subiquity to apply a home-grown Multipart MIME formatted user-data to a server install. Thanks in advance for your time, insights, and good work on this useful project!

I've been researching, experimenting and reverse engineering subiquity for the last 2 days trying to understand how to provide my own multipart MIME user-data to subiquity and have fallen short writing the user-data archive (along with its empty meta-data brother) to various folders in my custom iso, custom squashfs, custom initrd. These informed my most recent attempt which was to assume that subiquity doesn't support Multipart MIME _at all_ as a format, and try to embed the archive, as created by the following command:

`cloud-init devel make-mime -a "cloud-config.yaml:cloud-config" -a "some-script.sh:x-shellscript-per-once"`

under the `user-data:` clause of my `autoinstall.yaml` file, a la:

```
user-data: |
    Content-Type: multipart/mixed; boundary="===============9020930996497418650=="
    MIME-Version: 1.0

    --===============9020930996497418650==
    Content-Type: text/cloud-config; charset="utf-8"
    MIME-Version: 1.0
    Content-Transfer-Encoding: base64
    Content-Disposition: attachment; filename="./cloud-config.yaml"

    I2Nsb3VkLWNvbmZpZwpkaXNhYmxlX3Jvb3Q6IHRydWUKdGltZXpvbmU6IEFtZXJpY2EvQ2hpY2Fn
    bwpncm91cHM6CiAgLSBsaWJ2aXJ0CiAgLSBzdWRvCnVzZXJzOgogIC0gbmFtZTogdWJ1bnR1CiAg
    ICBwcmltYXJ5X2dyb3VwOiB1c2VycwogICAgZ3JvdXBzOgogICAgICAtIHN1ZG8KICAgICAgLSBs
    aWJ2aXJ0CiAgICAgIC0gbGlidmlydC1xZW11CiAgICAgIC0ga3ZtCiAgICBsb2NrX3Bhc3N3ZDog
.
.
.

    ZXQpIl0KCiMgcmVzdGFydCBhZnRlciBmaXJzdCBob3N0IGluaXRpYWwgcHJvdmlzaW9uaW5nCnBv
    d2VyX3N0YXRlOgogIGRlbGF5OiBub3cKICBtb2RlOiByZWJvb3QKICBtZXNzYWdlOiBSZWJvb3Rp
    bmcgbWFjaGluZQo=

    --===============9020930996497418650==
    Content-Type: text/x-shellscript-per-once; charset="utf-8"
    MIME-Version: 1.0
    Content-Transfer-Encoding: base64
    Content-Disposition: attachment; filename="./pfsense-vm.sh"

    IyEvYmluL2Jhc2gKIyBEZXBsb3kgcGZTZW5zZSBWaXJ0dWFsIE1hY2hpbmUgdXNpbmcga3ZtK3Fl
    bXUrbGlidmlydAojCiMgUmVxdWlyZXM6CiMgICAtICRVU0VSIGlzIHBhc3N3b3JkbGVzcyBzdWRv
    ZXIgb3IgYWJsZSB0byBpbnRlcmFjdGl2ZWx5IGF1dGhOIGFzIHN1ZG9lcgojICAgLSAkVVNFUiBp
.
.
.
    fSIgIiR7Vk1fRElTS19QQVRIfS5iYWsiCmVsc2UKICBsb2cgIkVSUk9SOiB2aXJ0LWluc3RhbGwg
    cmV0dXJuZWQgJWIiICIkPyIKICBleGl0IDEKZmkKCgo=

    --===============9020930996497418650==--
```

I'm now placing this `autoinstall.yaml` at the root of /cdrom in a (slightly-modified) liveCD iso. The result of this is a crash, indicating this format isn't understood by subiquity as valid `user-data`:

```
 2023-09-14 05:10:58,547 DEBUG subiquity.server.server:478 load_autoinstall_config only_early False file /autoinstall.yaml
 2023-09-14 05:10:58,556 DEBUG root:37 start: subiquity/Early/load_autoinstall_data:
 2023-09-14 05:10:58,556 DEBUG root:37 finish: subiquity/Early/load_autoinstall_data: SUCCESS:
 2023-09-14 05:10:58,557 DEBUG root:37 start: subiquity/Reporting/load_autoinstall_data:
 2023-09-14 05:10:58,557 DEBUG root:37 finish: subiquity/Reporting/load_autoinstall_data: SUCCESS:
 2023-09-14 05:10:58,557 DEBUG root:37 start: subiquity/Error/load_autoinstall_data:
 2023-09-14 05:10:58,557 DEBUG root:37 finish: subiquity/Error/load_autoinstall_data: SUCCESS:
 2023-09-14 05:10:58,557 DEBUG root:37 start: subiquity/Userdata/load_autoinstall_data:
 2023-09-14 05:10:58,558 ERROR root:37 finish: subiquity/Userdata/load_autoinstall_data: FAIL: 'Content-Type: multipart/mixed; boundary="===============9020930996497418650=="\nMIME-Version: 1.0\n\n--===============9020930996497418650==\nContent-Type: text/cloud-config; charset="utf-8"\nMIME-Version: 1.0\nContent-Transfer-Encoding: base64\nContent-Disposition: attachment; filename="/home/tim/src/du/elcamino/cloud-config.yaml"\n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
.
.
.
```