[Multipart] Support model format for @multipartBody

[msyyc]

• fixes part of [TCGC] support advanced feature of multipart #960
• Pending on [Http] expose necessary api for multipart microsoft/typespec#3676

Context:
Typespec supports 3 kinds of format to define multipart:

1. common

op upload(
  @header `content-type`: "multipart/form-data",
  @body body: {
    basic: string,
    headShots: bytes[],
  }
): void;

2. advanced model format

op upload(
  @header `content-type`: "multipart/form-data",
  @multipartBody body: {
    fullName: HttpPart<string>,
    headShots: HttpPart<bytes[]>
  }
): void;

3. advanced array format

op upload(
  @header `content-type`: "multipart/form-data",
  @multipartBody body: [
    // single
    HttpPart<string, #{ name: "fullName" }>,
    HttpPart<bytes[], #{ name: "headShots" }>,
  ]
): void;

• This PR is implementation for API design multipart api design (DO NOT MERGE) #987, mainly for 1 and 2 format. Format 3 will be implemented in another PR.

[azure-sdk]

All changed packages have been documented.

• ✅ @azure-tools/typespec-client-generator-core

Show changes

@azure-tools/typespec-client-generator-core - feature ✏️

Support model format of @multipartBody

@azure-tools/typespec-client-generator-core - fix ✏️

have @clientName work for operation groups as well

@azure-tools/typespec-client-generator-core - feature ✏️

add xml usage and change enumvalue arg representation in generic decorators

[azure-sdk]

You can try these changes at https://cadlplayground.z22.web.core.windows.net/typespec-azure/prs/1090/

Check the website changes at https://tspwebsitepr.z22.web.core.windows.net/typespec-azure/prs/1090/