Add SelectableBuyerAndSellerReportingIdsTruncateToKAnonLimit parameter.

This parameter is part of the
`FledgeLimitSelectableBuyerAndSellerReportingIdsFetchedFromKAnon`
feature, which enables the browser to limit the number of
`selectableBuyerAndSellerReportingIds` for which k-anonymity is fetched.
This new parameter helps reduce any potential confusion over which of
the `selectableBuyerAndSellerReportingIds` can be part of a bid that
could potentially be considered k-anonymous by only allowing those
limited set of `selectableBuyerAndSellerReportingIds` to be provided to
calls to `generateBid()`, but only loading the limited set of
`selectableBuyerAndSellerReportingIds`.

Change-Id: I44ca4c0a489bef6f130ef102f2e8efb1681b852f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6410131
Reviewed-by: Russ Hamilton <behamilton@google.com>
Commit-Queue: Orr Bernstein <orrb@google.com>
Cr-Commit-Position: refs/heads/main@{#1439726}