[REFACTOR] update cybersource and BOA connector transformers

[prasunna09]

Currently the from impl for card issuer of Cybersource lies in BOA. ref -

hyperswitch/crates/router/src/connector/cybersource/transformers.rs

Line 1267 in edd0998

Ok(issuer) => Some(String::from(issuer)),

Move this impl to cybersource transformers -

hyperswitch/crates/router/src/connector/bankofamerica/transformers.rs

Line 522 in edd0998

impl From<CardIssuer> for String {

also, instead of returning strings, can we have enum variant for this?
Please update accordingly in both the cybersource and boa transformers.