[SPARK-29664][PYTHON][SQL][FOLLOW-UP] Add deprecation warnings for getItem instead

[HyukjinKwon]

What changes were proposed in this pull request?

This PR proposes to use a different approach instead of breaking it per Micheal's rubric added at https://spark.apache.org/versioning-policy.html. It deprecates the behaviour for now. It will be gradually removed in the future releases.

After this change,

import warnings
warnings.simplefilter("always")
from pyspark.sql.functions import *
df = spark.range(2)
map_col = create_map(lit(0), lit(100), lit(1), lit(200))
df.withColumn("mapped", map_col.getItem(col('id'))).show()

/.../python/pyspark/sql/column.py:311: DeprecationWarning: A column as 'key' in getItem is 
deprecated as of Spark 3.0, and will not be supported in the future release. Use `column[key]` 
or `column.key` syntax instead.
  DeprecationWarning)
...

import warnings
warnings.simplefilter("always")
from pyspark.sql.functions import *
df = spark.range(2)
struct_col = struct(lit(0), lit(100), lit(1), lit(200))
df.withColumn("struct", struct_col.getField(lit("col1"))).show()

/.../spark/python/pyspark/sql/column.py:336: DeprecationWarning: A column as 'name'
in getField is deprecated as of Spark 3.0, and will not be supported in the future release. Use
`column[name]` or `column.name` syntax instead.
  DeprecationWarning)

Why are the changes needed?

To prevent the radical behaviour change after the amended versioning policy.

Does this PR introduce any user-facing change?

Yes, it will show the deprecated warning message.

How was this patch tested?

Manually tested.

[HyukjinKwon]

cc @imback82 and @viirya can you take a look please?

[SparkQA]

Test build #121749 has finished for PR 28327 at commit eb2d788.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[imback82]

LGTM

[SparkQA]

Test build #121863 has finished for PR 28327 at commit 3ec641d.

• This patch fails PySpark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[HyukjinKwon]

retest this please

[HyukjinKwon]

Thanks @viirya and @imback82.

[SparkQA]

Test build #121868 has finished for PR 28327 at commit 3ec641d.

• This patch fails PySpark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[HyukjinKwon]

retest this please

[SparkQA]

Test build #121872 has finished for PR 28327 at commit 3ec641d.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[HyukjinKwon]

Merged to master and branch-3.0.