Add 'Index.search' API wrapper.

…#1178)

* fix: disambiguate field headers whose names are reserved python words

* Add FieldHeader class

* update tests and templates

* chore(deps): update all dependencies

* pin google-cloud-bigquery===3.30.0 for python 3.8

---------

Co-authored-by: Lingqing Gan <lingqing.gan@gmail.com>

deps!: BigQuery Storage and pyarrow are required dependencies (#776)

fix!: use nullable `Int64` and `boolean` dtypes in `to_dataframe` (#786) 

feat!: destination tables are no-longer removed by `create_job` (#891)

feat!: In `to_dataframe`, use `dbdate` and `dbtime` dtypes from db-dtypes package for BigQuery DATE and TIME columns (#972)

fix!: automatically convert out-of-bounds dates in `to_dataframe`, remove `date_as_object` argument (#972)

feat!: mark the package as type-checked (#1058)

feat!: default to DATETIME type when loading timezone-naive datetimes from Pandas (#1061)

feat: add `api_method` parameter to `Client.query` to select `INSERT` or `QUERY` API (#967)

fix: improve type annotations for mypy validation (#1081)

feat: use `StandardSqlField` class for `Model.feature_columns` and `Model.label_columns` (#1117)

docs: Add migration guide from version 2.x to 3.x (#1027)

Release-As: 3.0.0

Source-Link: googleapis/synthtool@febaccc
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-python:latest@sha256:caffe0a9277daeccc4d1de5c9b55ebba0901b57c2f713ec9c876b0d4ec064f61

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>