Fix #3263: Adding base64 file upload component

[alexcottner]

Resolves #3263.
Adding a base64 encoding file upload option. See demo video below for how this prototype works.

This PR:

1. Adds a new RJSF widget to allow files to be "uploaded" as bse64 strings
2. Updates the Read-me with an example of this (should we update remote-settings docs too?)
3. Adds unit tests for the new component
4. Adds some validation to prevent too-large files from being uploaded (1MB cap)

See the demo video and screenshots below.

simplescreenrecorder-2025-02-12_12.13.19.mp4

Validation error for too large files:

[alexcottner]

@leplatrem and @nathan-barrett - How does this feel?

[leplatrem]

Nice!
micronit: I would maybe file in the widget name? base64file?

[alexcottner]

This mock may be worth committing to memory. This was the most difficult thing in the PR to develop.

[nathan-barrett]

Awesome getting this in, lgtm

[leplatrem]

Nice! Thanks Alex for working on this.

I would recommend adding a test that does an assertion on the string field value that we obtain (eg data:content/type;base64,)

Important

We should probably add info about this in the records vs attachment section of our RS docs https://remote-settings.readthedocs.io/en/latest/getting-started.html#records-vs-attachments

[leplatrem]

nit: it's called onChange elsewhere

[alexcottner]

Yeah I think I was just avoiding that as we were getting a prop from rjsf called onChange. But I can just rename that one to rjsfOnChange.

[leplatrem]

then forget it 👌

[alexcottner]

Nah, already did the thing

[leplatrem]

Renamed the PR title, because:

➜  kinto-admin git:(master) ✗ git log | grep -i 'fix' | wc -l
     296
➜  kinto-admin git:(master) ✗ git log | grep -i 'issue' | wc -l
      11