Bug 34764: Set fee_acknowledged to expect a string

Bug 34764: Set fee_acknowledged to expect a string

For consistency with other options I opted to have the fee acknowledged
parameter to expect a string and as it's an optional parameter I've
dropped the default value of 'N' too.

Test plan
1) Prior to this patch
1a) Attempt a checkout without passing -fa/--fee_acknowledged flag
    `./misc/sip_cli_emulator.pl -a localhost -p 6001 -su term1 -sp term1 -l CPL --patron 23529000035676 --item 39999000007756 -m checkout`
    The final field of the SIP request will be '|BON', the default
1b) Attempt a checkout passing -fa/--fee_acknowledged flag
    `./misc/sip_cli_emulator.pl -a localhost -p 6001 -su term1 -sp term1 -l CPL --fee-acknowledged --patron 23529000035676 --item 39999000007756 -m checkout`
    The final field of the SIP request will still be '|BON', failure
1c) Attempt a checkout passing a string for fee_acknoewledeged flag
    `./misc/sip_cli_emulator.pl -a localhost -p 6001 -su term1 -sp term1 -l CPL --fee-acknowledged Y --patron 23529000035676 --item 39999000007756 -m checkout`
    The final field of the SIP request will still be '|BON', failure
2) Apply the patch
2a) Attempt a checkout without passing -fa/--fee_acknowledged flag
    `./misc/sip_cli_emulator.pl -a localhost -p 6001 -su term1 -sp term1 -l CPL --patron 23529000035676 --item 39999000007756 -m checkout`
    The optional `|BO` element should not be present
2b) Attempt a checkout passing -fa/--fee_acknowledged flag
    `./misc/sip_cli_emulator.pl -a localhost -p 6001 -su term1 -sp term1 -l CPL --fee-acknowledged N --patron 23529000035676 --item 39999000007756 -m checkout`
    The final field of the SIP request will now be '|BON', success
2c) Attempt a checkout passing a string for fee_acknoewledeged flag
    `./misc/sip_cli_emulator.pl -a localhost -p 6001 -su term1 -sp term1 -l CPL --fee-acknowledged Y --patron 23529000035676 --item 39999000007756 -m checkout`
    The final field of the SIP request will now be '|BOY', success

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit 0e9603bb49436e4bb74e893f74a63ef31065f798)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>