Allow more BIP0022 fields in GetBlockTemplateCmd.

BIP0022 defines optional fields in a getblocktemplate request for long
polling and template tweaking.

In addition, for template tweaking, there are two fields, sigoplimit and
sizelimit, which are atypical in that they are allowed to be either
booleans or numeric.  This requires the fields to be represented as
interfaces which means any code making use of the struct will need to use
type assertions or a type switch.

This commit updates GetBlockTemplateCmd accordingly.

ok @jcvernaleo