Add initial Adaptive Card Table support

OVERVIEW

Add early implementation of Adaptive Card Table support.

CHANGES

- Expand base/shared Element type with fields specific to the
  Table type
- Refresh documentation for Column type to note that it serves a
  dual role
  - member of a ColumnSet
  - member of a Table (TableColumnDefinition)
- Add new types specific to tables
- Examples
  - add example of manually creating a table
  - add example of creating a table with headers
  - add example of creating a table of unordered "grid" values
- README
  - Add references to new examples
  - Add Table element documentation reference

LIMITATIONS

Support for Adaptive Card Table elements is new. There are likely
sharp edges and missing support. Feedback is welcome.

REFERENCES

- refs GH-205
- https://adaptivecards.io/explorer/Table.html

README.md

@@ -27,6 +27,7 @@ A package to send messages to a Microsoft Teams channel.
  - [Examples](#examples)
  - [Basic](#basic)
  - [User Mention](#user-mention)
+ - [Tables](#tables)
  - [Set custom user agent](#set-custom-user-agent)
  - [Add an Action](#add-an-action)
  - [Disable webhook URL prefix validation](#disable-webhook-url-prefix-validation)
@@ -201,6 +202,15 @@ is not available in the legacy `MessageCard` card format.
 - File: [user-mention-verbose](./examples/adaptivecard/user-mention-verbose/main.go)
  - this example does not necessarily reflect an optimal implementation
 
+#### Tables
+
+These examples illustrates the use of a [`Table`][adaptivecard-table]. This
+feature is not available in the legacy `MessageCard` card format.
+
+- File: [table-manually-created](./examples/adaptivecard/table-manually-created/main.go)
+- File: [table-unordered-grid](./examples/adaptivecard/table-unordered-grid/main.go)
+- File: [table-with-headers](./examples/adaptivecard/table-with-headers/main.go)
+
 #### Set custom user agent
 
 This example illustrates setting a custom user agent.
@@ -280,3 +290,4 @@ using either this library or the original project.
 [adaptivecard-ref]: <https://adaptivecards.io/explorer>
 [adaptivecard-ref-actions]: <https://docs.microsoft.com/en-us/adaptive-cards/authoring-cards/getting-started>
 [adaptivecard-user-mentions]: <https://docs.microsoft.com/en-us/microsoftteams/platform/task-modules-and-cards/cards/cards-format#mention-support-within-adaptive-cards>
+[adaptivecard-table]: <https://adaptivecards.io/explorer/Table.html>