Fix whitelist monitor path and disabled flags

[omrozowicz-splunk]

This PR:

• Fix whitelist/blacklist handling for monitor and batch stanzas: Splunk TA default stanzas typically set whitelist to a PCRE regex (e.g. (\.log|log$|messages|secure|auth)). Previously this regex was joined to the monitored path and passed to filelog as a glob, which never matched anything. Now whitelist/blacklist values are always treated as PCRE regexes per Splunk docs and applied as stanza filter operators on log.file.path.
• Fix directory paths without whitelist: A monitor stanza pointing at a plain directory with no whitelist param now correctly expands to dir/* so filelog can match files inside it.
• Fix disabled=true not being recognised: IsDisabled() previously only handled disabled=1; it now also handles disabled=true.
• Deduplicate shared logic: Extracted ApplyIncludeExclude and ApplyStanzaConfig into a shared pkg/splunkta/receiver/filter package used by both monitorreceiver and batchreceiver.

[dhruvanshus-crest]

this will return true for some regex which can include *, [.

For example, .*log, foo.*bar, and [0-9]+ are valid regular expressions, but this function returns true for them. They are then passed to filelog as globs and no regex filter is added.

[omrozowicz-splunk]

from splunk's documentation looks like whitelist is:

whitelist = <your_custom regex>

so I think we can assume it is always a regex