Support scalar index

[congqixia]

For Scalar index support:

• Add scalarIndex type
• Remove field type check for most index usage
• Fix index example code
• Fix connect logic error
  /kind improvement
  See also [Enhancement]: Create Scalar Index #497

[codecov]

Codecov Report

Merging #498 (eb5427c) into v2.2.x (f399a77) will decrease coverage by 0.20%.
The diff coverage is 13.33%.

@@            Coverage Diff             @@
##           v2.2.x     #498      +/-   ##
==========================================
- Coverage   80.65%   80.46%   -0.20%     
==========================================
  Files          34       35       +1     
  Lines        2812     2820       +8     
==========================================
+ Hits         2268     2269       +1     
- Misses        432      438       +6     
- Partials      112      113       +1     

Impacted Files	Coverage Δ
client/grpc_client.go	47.50% <0.00%> (ø)
entity/index.go	100.00% <ø> (ø)
entity/index_scalar.go	0.00% <0.00%> (ø)
client/index.go	76.92% <20.00%> (-1.03%)	⬇️
client/data.go	68.85% <50.00%> (ø)

[yah01]

/lgtm

[sre-ci-robot]

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