Add entry-related methods to scoreboard API. Adds BUKKIT-5779

##### The Issue:

Most methods in the scoreboard API which pertain to OfflinePlayer have been deprecated in favor of the use of entries. However, some methods remain without an entry-related alternative.

##### Justification:

If OfflinePlayers in relation to the scoreboard API are to be deprecated, the change should be thorough and offer an alternative for all methods.

##### PR Breakdown:

Implement getEntryTeam(String) in Scoreboard.class
Implement getEntries(), addEntry(String), removeEntry(String), and hasEntry(String) in Team.class

##### Testing Results and Materials:

The new scoreboard API methods function as expected when employed in a basic plugin.

##### Relevant PRs:

[B-1108](https://github.com/Bukkit/Bukkit/pull/1108)

##### JIRA Ticket:

[BUKKIT-5779](https://bukkit.atlassian.net/browse/BUKKIT-5779)