AST-2022-001 - res_stir_shaken/curl: Limit file size and check start.

AST-2022-001 - res_stir_shaken/curl: Limit file size and check start.

Put checks in place to limit how much we will actually download, as well
as a check for the data we receive at the start to ensure it begins with
what we would expect a certificate to begin with.

ASTERISK-29872

Change-Id: Ifd3c6b8bd52b8b6192a04166ccce4fc8a8000b46