Allow rebase with a secondary email via REST

Update the rebase REST API to add a new optional 'committer_email'.
It provides the flexibility to choose a secondary email of the
current user or latest uploader (when on_behalf_of_uploader=true)
instead of their preferred email to perform the rebase. This option
is not supported when rebasing a chain of changes. UI will be updated
in a followup change.

Release-Notes: rebase can be performed with a secondary email using REST API
Change-Id: I4007b18b460fdd341fa0abc205ea861634575a44
diff --git a/Documentation/rest-api-changes.txt b/Documentation/rest-api-changes.txt
index ea95680..5026c5f 100644
--- a/Documentation/rest-api-changes.txt
+++ b/Documentation/rest-api-changes.txt
@@ -1472,6 +1472,9 @@
 Optionally, the parent revision (of the oldest ancestor to be rebased) can be changed to another
 change, revision or branch through the link:#rebase-input[RebaseInput] entity.
 
+Providing a `committer_email` through the link:#rebase-input[RebaseInput] entity is not supported
+when rebasing a chain.
+
 If the chain is outdated, i.e., there's a change that depends on an old revision of its parent, the
 result is the same as individually rebasing all outdated changes on top of their parent's latest
 revision before running the rebase chain action.
@@ -8343,6 +8346,10 @@
 In addition, rebasing on behalf of the uploader is only supported for the
 current patch set of a change. +
 If the caller is the uploader this flag is ignored and a normal rebase is done.
+|`committer_email`|optional|
+Rebase is committed using this email address. Only the registered emails
+of the calling user or uploader (when `on_behalf_of_uploader` is `true`) are
+considered valid. This option is not supported when rebasing a chain.
 |`validation_options`   |optional|
 Map with key-value pairs that are forwarded as options to the commit validation
 listeners (e.g. can be used to skip certain validations). Which validation