Support providing binary file contents in base64 encoding

[rohanku]

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Is your feature request related to a problem? Please describe.
I am using release-please and need to overwrite binary files in my release flow. release-please uses code-suggester to create PRs.
Describe the solution you'd like
Simply add a field to FileData to declare whether the content string provided is in utf-8 or base64 encoding. I already have code for this (~ 20 lines of code) and can make a PR.
Describe alternatives you've considered
It is a fairly simple fix, not sure if there are any other ways to do it.
Additional context
N/A