[RLC] Support for new TypeSpec multipart capabilities

[timovv]

Update RLC multipart/form-data implementation to add support for new TypeSpec capabilities introduced in microsoft/typespec#3046. This includes:

• Allowing filename to be optional/required
• Allowing different multipart content types e.g. multipart/mixed
• Custom part headers

Among other things TBD

[qiaozha]

@timovv can you also help take a look at the tcgc pr for Azure/typespec-azure#987 mfd api ?

I think for RLC, it's fine, but for Modular, we need to consider to use the api? probably just make sure everything we need is there.