fix: Remove metadata fields in verb builders

[sitaram-kalluri]

- What I did

• Remove AtKey fields and metadata fields from the verb builders. Use AtKey instance in "AbstractVerbBuilder" to set the key.
• Since "key" in AtKey cannot be null, removed "?" which indicates that key can be null, and marked it as late.
• Removed static const String atOperation = 'operation from "at_constants.dart" file because it is redundant.
• Remove "?" for "isBinary" and "isEncrypted" fields default them to false.
• In AtClient pacakge, AtKey.Metadata is verififed for null check and if null assigned to Metadata() . Initialized Metadata field to remove the null check in at_client methods.
• Since metadata is removed from verb builders, removed packages/at_commons/lib/src/verb/metadata_using_verb_builder.dart file which is not needed.
• Uptake the at_commons changes into at_lookup.
• Modify the tests in at_commons.

Raised draft PR's in at_client_sdk and at_server to validate the changes:

at_server: atsign-foundation/at_server#1688

at_client: atsign-foundation/at_client_sdk#1179

[murali-shris]

in verb builders replace string concatenation with string builder

[murali-shris]

at_lookup changes you can move to separate PR.
at_lookup dependency on at_commons 4.x is a minor version release for at_lookup?

[sitaram-kalluri]

• Sure Murali, WIll move the at_lookup changes to a new PR.
• At_lookup can pusblished as a minor version because we are not introducing any breaking changes.

[sitaram-kalluri]

in verb builders replace string concatenation with string builder

This change is completed and pushed the changes.