Add sharding function section to ddl schema

[AnaNek]

RFC - tarantool/crud#27 (comment)

[AnaNek]

Sharding function API in ddl schema

With body field

Description: sharding_func section can contain string value or table value. String value is for vshard sharding function identifier like strcrc32/mpcrc32, for sharding function specified as dot.notation.func and for sharding function name from _G. Custom sharding function can be passed as Lua code as table with body field:

sharding_func = 'mpcrc32' | 'strcrc32' | 'dot.notation.func' | {body = 'function(key) return <...> end'},
    <...>

Pros:

1. Similar API exists: box.func = {body = “...”};
2. Allows to differentiate whether it is an executable function or an identifier.

Cons:

1. API looks more complicated.

Without body field

Description: sharding_func section can contain only a string value.

sharding_func = 'mpcrc32' | 'strcrc32' | 'dot.notation.func' | 'function(key) return <...> end',  <...>

Pros:

1. API looks simpler.

Cons:

1. The definition of the function name / function body is implicit, in general, such API looks like a potential security problem (but it is important for us only because of the desire to make api similar to existing ones in the future).

[unera]

I think the second variant is ok.

[Totktonada]

@AnaNek Thank you for the proposals!

There were several chatting rounds around those proposals and the short summary is the following.

1. We'll not implement shortcuts 'mpcrc32' and 'strcrc32' for now.
2. We'll use the {body = 'function(key) return <...> end'} syntax (the first variant).