[Smyatkin-Maxim]

What does this PR do?

Allows LOCATION ... ON MASTER syntax for file:// protocol in external tables.

Additional Context

The syntax was already there but it is not supported for any ext table protocols except CUSTOM. I've added support for local files. My motivation is use this mechanism as yet another way to read logs from the local filesystem. It's already possible with EXECUTE in external web tables, but for security reasons we prohibit it completely in our installations.
Something like this:

CREATE READABLE EXTERNAL TABLE pg_log_tbl_master 
(                                    
    logtime timestamp with time zone,
    loguser text,
    logdatabase text,
    logpid text,
    logthread text,
    loghost text,
    logport text,                           
    logsessiontime timestamp with time zone,
    logtransaction int,
    logsession text,
    logcmdcount text,
    logsegment text,
    logslice text,
    logdistxact text,
    loglocalxact text,
    logsubxact text,
    logseverity text,
    logstate text,
    logmessage text,
    logdetail text,
    loghint text,
    logquery text,
    logquerypos int,
    logcontext text,
    logdebug text,
    logcursorpos int,
    logfunction text,
    logfile text,
    logline int,
    logstack text
)                                                                      
LOCATION (
    'file://my_local_cloudberry_build/gpAux/gpdemo/datadirs/dbfast1/demoDataDir0/pg_log/*.csv' 
) ON MASTER
FORMAT 'CSV' (DELIMITER AS ',' NULL AS '' QUOTE AS '"');