Add support for specifying field name aliases

[Alfus]

Using a new custom option:

message SchemaMessage {
  string string_schema_field = 1 [(buf.schema.field).alias = "string_schema_field_alias"];
  int32 int32_schema_field = 2 [
    (buf.schema.field).alias = "int32_schema_field_alias",
    (buf.schema.field).alias_json = "int32 schema field alias json"
  ];
}

[bufdev]

I need more explanation here before we add a public repo to the bufbuild org. What is this for?

[Alfus]

The main use for this is protoyaml. It allows a field to be renamed without atomically updating all yaml files. Beyoned that, its a key missing feature for any usage of names in a serialization formats (e.g. json, yaml, flags, protobuf with optional ids, etc). For these formats, the main field name is preferred, and aliases support backwards compatibility for reads.