Make Iceberg support case insensitivity

[xabriel]

Iceberg's current implementation has column case sensitivity, which hinders usability, as most sql users expect case insensitivity by default. While a query like the following will succeed in other Spark Readers, it will fail on Iceberg:

SELECT COUNT(*)
FROM iceTable
WHERE year = 2017
  AND MONTH = 11 -- Notice how MONTH has different casing than other predicates
  AND day = 01

This will fail with a stack trace similar to:

com.google.common.util.concurrent.UncheckedExecutionException: com.netflix.iceberg.exceptions.ValidationException: Cannot find field 'MONTH' in struct: struct<...>
...

PR to solve this issue at iceberg-api level: #82

More PRs to use this new flag to follow.

[xabriel]

PR #82 solves this issue at iceberg-api level.

We still need follow up PRs to:

1. Expose this new caseSensitive flag as a configuration, perhaps by introducing the use of org.apache.hadoop.conf.Configuration.
2. Also need to address a comment from @rdblue on Make expression binding support a case sensitivity flag #82:

I think some of these Evaluators will also need case sensitivity options. It doesn't do much good to support it in expression binding if it isn't also exposed when working with expressions in other ways. Can you also open a follow-up issue?

[xabriel]

PR #89, just merged, solves this problem all the way to the Spark Reader.

Jotting here some minor follow up items so that we don't forget:

Need to address comments:
#89 (comment)
and
#89 (comment)

[xabriel]

There's another issue with Filterables described in #145 .

[rdblue]

I'm going to close this because #89 and #141 fixed the original problem. #145 would be nice to fix, but the code works as it is right now.