[DPE-2695] + [partial DPE-2700] POC Identify primary shard + Prevent removal of Primary shards

[MiaAltieri]

Issue

Primary shards are not identified, this can pose problems if removing a primary shard

Solution

Identify primary shards and prevent their removal

Follow up PR

move databases that use a primary shard to a different shard if removing primary (I don't want to add this to this PR as to reduce the complexity in the review)

Testing

As this is a POC there are no integration tests added, instead tests were performed by hand

# deploy shards + config server
juju deploy ./*charm --config role="config-server" config-server-one 
juju deploy ./*charm --config role="shard" shard-one 
juju deploy ./*charm --config role="shard" shard-two 

# relate shards
juju integrate config-server-one:config-server shard-one:sharding
juju integrate config-server-one:config-server shard-two:sharding

# write data to shard-one
juju ssh config-server-one/0
charmed-mongodb.mongosh <URI>
use test_db
db.createCollection("cool_animals")
db.cool_animals.insertOne({"horses":"unicorn"})
sh.enableSharding("test_db")

# write data to shard-two
use test_db_2
db.createCollection("cool_animals")
db.cool_animals.insertOne({"horses":"unicorn"})
sh.enableSharding("test_db")
db.adminCommand( { movePrimary : "test_db_2", to : "shard-one" } )

# show both shards
use config 
show collections 
db.databases.find()

# exit
exit
exit

# remove shard and verify error in config server and forever wait in shard 
juju remove-relation config-server-one:config-server shard-two:sharding
juju status --watch 1s