Allow table providers to indicate their type for catalog metadata

[returnString]

Which issue does this PR close?

Closes#191.

Rationale for this change

This allows registered TableProviders to approximately indicate the mechanism used to retrieve data during query execution, which in turn allows tables to be logged with a table_type column in information_schema.tables. This was previously partially supported by classifying all registered tables as BASE TABLE and all generated system tables as VIEW.

This means that DataFusion consumers can now build systems that allow end-users to differentiate between relation types. For example, you could build a TableProvider that stores a query plan, use this as a view in your execution context, and end users would be aware that any queries over it in turn trigger a query internally. This is also often used to split relations into different lists for tables vs views in GUI/IDE setups.

What changes are included in this PR?

• Added TableType enum
• Exposed new trait method, TableProvider::table_type
• Updated information_schema builder to use new table types

Are there any user-facing changes?

TableProvider::table_type is introduced as a new trait method, but comes with a default implementation returning TableType::Base to preserve back-compat.

[codecov-commenter]

Codecov Report

Merging #205 (08e0044) into master (ddaea81) will decrease coverage by 0.01%.
The diff coverage is 72.22%.

❗ Current head 08e0044 differs from pull request most recent head 589d8fa. Consider uploading reports for the commit 589d8fa to get more accurate results

@@ Coverage Diff @@## master #205 +/- ##
==========================================
- Coverage 76.35% 76.33% -0.02% 
==========================================
Files 134 134 Lines 23018 23042 +24 ==========================================
+ Hits 17576 17590 +14 - Misses 5442 5452 +10 

Impacted Files	Coverage Δ
datafusion/src/execution/context.rs	92.63% <68.75%> (-0.31%)	⬇️
datafusion/src/catalog/information_schema.rs	86.72% <72.22%> (-2.06%)	⬇️
datafusion/src/datasource/datasource.rs	100.00% <100.00%> (ø)

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[alamb]

looks great to me. Thanks @returnString

I plan to leave this open for a bit to see if anyone else has comments, but I think it is ready to go

[alamb]

A preview of things to come, perhaps :)