rgw: rest and http client code to use param vectors

rgw: rest and http client code to use param vectors

Replaces param/header lists with vectors. In these cases, we're only ever
adding to the back of the list, so a vector should be more efficient.
Also moves param_pair_t/param_vec_t higher up the include chain for
cleaner function signatures.

Signed-off-by: Karol Mroz <kmroz@suse.com>