idl: allow to parse `const` specifiers for template arguments
idl: allow to parse `const` specifiers for template arguments

This patch introduces very limited support for declaring `const`
template parameters in data members.

It's not covering all the cases, e.g.
`const type member_variable` and `const template_def<T1, T2, ...>`
syntax is not supported at the moment.

Though the changes are enough for raft-related use: this makes it
possible to declare `std::vector<raft::log_entries_ptr>` (aka
`std::vector<lw_shared_ptr<const raft::log_entry>>`) in the IDL.

Existing IDL files are not affected in any way.

Signed-off-by: Pavel Solodovnikov <2cf5887fafb117b7ff3e29b9720a7305c3313494@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla