Feature request - UI support for separate migration shard id

[darshan-sj]

Support for separate migration project id and spanner project id was added in this PR.

Need to add support in the SMT UI to take in Migration project id separately from the prepare migrations page. The code support for these two fields is added in webv2/web.go and config file handlers. Pending task is to take in the migration project id (if it is different from the spanner project id) in prepare migrations page and pass it to backend.

Pending tasks:

• UX design to take in migration project id with fallback to spanner project id
• UI implementation
• API changes to pass the migration project id.