Support maxProperties=1 constraints by marking them as oneof for protobuf generation

[lucy66hw]

Description

Support maxProperties=1 constraints by marking them as oneof for protobuf generation

Test

lucy66hw#63
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[karenyrx]

Pasting convo from slack for reference:

This is my understanding of the expected behavior of min/max=1 (let me know if i got this right)

1. case 1: if min/max properties=1 exists, and has properties but no additionalProperties (e.g. for SpanQuery)
   -> then we wrap the properties inside a proto oneof
2. case 2: if min/max properties=1 , and has additionalProperties , but no properties (e.g. fuzzy) ,
   -> then we convert to string field instead of map<> with 1 element
3. case 3: if min/maxproperties=1 and has both additionalProperties and properties (e.g. TermsQuery)
   -> then the min/max=1 applies for the properties field not the additionalProperties
   just wondering which case (1, 2, or 3) this PR implements, or all 3

This PR is to implement case 1. the case 2 had already implemented previously. it already changed the schema to not having additionalProperty. for case3, it would be the corner case that we can't handle it properly.

Summary:

• Case 2 has a separate rule to take care of it, before case 1/3 rules are appied.
• This PR implements case 1
• Throw error for case 3

[karenyrx]

Removing minProperties=1 in proto makes sense since there is no way to represent this in protos, and should be validated in server-side code instead.