[mlir][linalg] Support parsing attributes in named op spec

With this, now we can specify a list of attributes on named ops
generated from the spec. The format is defined as

```
attr-id ::= bare-id (`?`)?
attr-typedef ::= type (`[` `]`)?
attr-def ::= attr-id `:` attr-typedef

tc-attr-def ::= `attr` `(` attr-def-list `)`
tc-def ::= `def` bare-id
  `(`tensor-def-list`)` `->` `(` tensor-def-list`)`
  (tc-attr-def)?
```

For example,

```
ods_def<SomeCppOp>
def some_op(...) -> (...)
attr(
  f32_attr: f32,
  i32_attr: i32,
  array_attr : f32[],
  optional_attr? : f32
)
```

where `?` means optional attribute and `[]` means array type.

Reviewed By: hanchung, nicolasvasilache

Differential Revision: https://reviews.llvm.org/D94240