Maintainers.txt: Add .pytool maintainers to BaseTools/Plugin

Plugins can be placed either in .pytool/Plugin (CI plugin) or
BaseTools/Plugin (build plugin).

Since most of the .pytool maintainers already review many of the
plugins placed there, the same maintainers are added for the Plugin
directory in BaseTools to increase the total number of maintainers
for plugin changes.

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Rebecca Cran <rebecca@bsdio.com>
Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Yuwei Chen <yuwei.chen@intel.com>
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>

Maintainers.txt

@@ -175,6 +175,13 @@ M: Liming Gao <gaoliming@byosoft.com.cn> [lgao4]
 R: Bob Feng <bob.c.feng@intel.com> [BobCF]
 R: Yuwei Chen <yuwei.chen@intel.com> [YuweiChen1110]
 
+BaseTools: Plugins
+F: BaseTools/Plugin/
+M: Sean Brogan <sean.brogan@microsoft.com> [spbrogan]
+M: Michael Kubacki <mikuback@linux.microsoft.com> [makubacki]
+R: Michael D Kinney <michael.d.kinney@intel.com> [mdkinney]
+R: Liming Gao <gaoliming@byosoft.com.cn> [lgao4]
+
 CryptoPkg
 F: CryptoPkg/
 W: https://github.com/tianocore/tianocore.github.io/wiki/CryptoPkg