Missing support for search_as_you_type in dynamic templates

[robertlyson]

Hello 👋,

NEST/Elasticsearch.Net version: 7.4.1

Elasticsearch version: 7.3.0

Description of the problem including expected versus actual behavior:

When creating dynamic template with fluent syntax there is no way to add mapping for search_as_you_type field type.

Steps to reproduce:

var createIndexResponse = await client.Indices.CreateAsync("index", o => o
    .Map<Act>(m => m
        .AutoMap<Act>()
        .DynamicTemplates(d => d
            .DynamicTemplate("stringassearch", dt => dt
                .PathMatch("entity.*")
                .MatchMappingType("string")
                .Mapping(ma => ma.SearchAsYouType(s => s))))));

It won't compile right now. Issue spotted in this SO question.

Provide ConnectionSettings (if relevant):

Provide DebugInformation (if relevant):

Describe the feature:

[russcam]

Thanks @robertlyson, I've opened #4259