Improve AccoutsFile::appends_account() return value format

[yhchiang-sol]

Problem

AccoutsFile::append_accounts() currently returns Vec.
This requires all the AccountsFile implementations to report the storage size
of each account. This information will be later used for tracking the alive-bytes
and shrinking process. However, not all the AccountsFile implementations
know the "exact" storage size of each account as the file could have shared meta-
data and other storage usage as mentioned in #34929 (comment).

Proposed Solution

Instead, at the time of writing accounts, accounts-db only needs to know the total size.
And it is not until the later stage we need an estimation of how much storage each account
occupies. As a result, it is better to have append_accounts() return the total file size, and
have a separate API to provide an estimation about how much storage size each account occupies.