More Sign In changes

[jordaaash]

This PR targets the alpha branch.

• Adjust sign in message generation and parsing to allow LF chars in statements and optional trailing LF chars.

[jordaaash]

The effect of this change is to build and parse the following kinds of messages:

No statement, no fields, optional terminating LFs:

solana.com wants you to sign in with your Solana account:
XYZ

Statement, no fields, optional terminating LFs:

solana.com wants you to sign in with your Solana account:
XYZ

This is a statement

Statement, no fields, multiple lines, optional terminating LFs:

solana.com wants you to sign in with your Solana account:
XYZ

This is a statement

It has multiple lines

Statement, multiple lines, fields:

solana.com wants you to sign in with your Solana account:
XYZ

This is a statement

It has multiple lines

URI: foo.com