Easier access to the agent's addresses

[Adamantios]

In the current implementation, a dev needs to use the string values of the ledgers in order to access a non-default address of the agent. This is error-prone and needs code investigation to figure it out.

A potential solution would be to:

1. Make the identifiers public in aea/configurations/constants.py.
2. Optionally implement properties in the SkillContext, such as:

@property
def sol_agent_address(self) -> str:
    """Get the agent's Solana address."""
    return self.agent_addresses[SOLANA_IDENTIFIER]